1. Field of Invention.
This invention relates to an electromagnetic flow meter converter (hereinafter called "converter") which amplifies and converts a deviation of a feedback signal from an input signal, into a frequency signal, and further converts the frequency signal into the feedback signal; and more particularly, to improvements in the span setting portion of the converter.
2. Description of Prior Art.
FIG. 1 shows an example of a conventional converter which employes a scheme to attain compatibility between the converter and a detector used therewith. In FIG. 1, to an input terminal 10, an input signal e.sub.i relating to a flow rate, is applied, which input signal is provided from an electrode of a detector (not shown). This input signal e.sub.i is amplified by an amplifier 11. The amplifier 11 has a variable gain so that a meter constant for compensation of variations of the characteristics of the detector can be set in the converter. A deviation of a feedback signal e.sub.f produced by a multiplier 12 from the output of amplifier 11 is amplified by a deviation (i.e. differential)amplifier 13. The output of deviation amplifier 13 is synchronous rectified by a synchronous rectifying circuit 14, into a DC voltage. This DC voltage is converted by a voltage-to-frequency converting circuit 15, into a frequency signal which has a certain pulse duration and whose frequency corresponds to the value of the DC voltage. This frequency signal is converted by a frequency-to-current converting circuit 16 into a DC current, which is outputted through an output terminal 17. The frequency signal from circuit 15 is also concurrently supplied to a frequency ratio converting circuit 18.
The frequency ratio converting circuit 18 comprises a delay circuit, frequency ratio selecting switch, counter, gate, etc, such as shown, for example, in Japanese Patent Publication 56-41944, "Signal Converter". The frequency output of frequency ratio converting circuit 18 is supplied to multiplier 12. A comparison signal e.sub.r, which is proportional to an excitation current, is produced across a resistor connected in series with an exciting coil of the detector (not shown). This comparison signal e.sub.r is applied to multiplier 12 through a terminal 19. A multiplier 12 is formed, for example, by switching elements having turn-on/off action which is controlled according to the frequency output of frequency ratio converting circuit 18, and provides feedback signal e.sub.f which is proportional to the product of comparison signal e.sub.r and the frequency output.
The operation of the converter of FIG. 1 is as follows. Denoting the input frequency of frequency ratio converting circuit 18 by F.sub.i1, the output frequency by F.sub.o1, and a frequency dividing factor (a ratio) by K.sub.1, the relation EQU F.sub.o1 =K.sub.1 F.sub.i1
is obtained. Because comparison signal e.sub.r is sampled by multiplier 12 in accordance with output frequency F.sub.o1, feedback signal e.sub.f can be written as EQU e.sub.f =F.sub.o1 e.sub.r =K.sub.1 F.sub.i1 e.sub.r ( 1)
with m.sub.f representing the meter constant, a coefficient 1/m.sub.f for compensation of variations of a signal voltage relating to a flow rate to be detected by the detector, has been set in amplifier 11, so that the output of the amplifier 11 becomes e.sub.i /m.sub.f.
Because the circuit loop is designed as a whole so that output e.sub.i /m.sub.f of amplifier 11 coincides with the feedback signal e.sub.f, the following is obtained ##EQU1##
Letting the relation between frequency F.sub.i1 and a current output I.sub.o1 at the output terminal 17 be EQU I.sub.o1 =.alpha.F.sub.i1 ( 4)
wherein .alpha. is a conversion constant, the following is obtained ##EQU2##
Accordingly, by setting, in the respective cases, a set of frequency division factor K.sub.1 and meter constant m.sub.f, it is possible to change the extent of the span, while preserving compatibility between the converter and the detector.
FIG. 2 shows another example of a conventional converter which uses a scheme for permitting zero adjustment. In FIG. 2, input signal e.sub.i is amplified by an amplifier 20 and applied to an inverting input end (-) of a deviation amplifier 22 through a resistor 21 for span adjustment. Between inverting input (-) and the output end of deviation amplifier 22, a parallel circuit is connected, comprising a condenser C.sub.1 and resistor R.sub.1, and functions so as to smooth the input voltage. On the other hand, a noninverting input end (+) of deviation amplifier 22 is connected to a common potential COM. The output end of amplifier 22 is connected to a synchronous rectifying circuit 23, so that a signal is synchronous rectified in synchronous rectifying circuit 23 and applied to an integrator 24.
The output of integrator 24 is converted by a voltage-to-frequency converting circuit 25, into a frequency signal F.sub.i2. This frequency signal F.sub.i2 is applied to a pulse width circuit 26 by which its pulse duration is made uniform, and serves to turn ON and OFF a switch SW1.
The comparison voltage e.sub.r is applied to one end of switch SW1 and switches ON and OFF in accordance with the output pulse of pulse width circuit 26. The resultant output is fed back negatively through a resistor 27 to inverting input (-) of deviation amplifier 22, so that the input voltage of deviation amplifier 22 will become zero. Accordingly, there is obtained an output whose frequency F.sub.i2 corresponds to input signal e.sub.i. This frequency signal F.sub.i2 is converted into a current output I.sub.o2 by, for example, a frequency to current converting circuit 28 and appears at output terminal 17. A zero point adjustment can be made with respect to a span being set, by voltage dividing comparison voltage e.sub.r and applying the obtained voltage through a resistor 29 for zero point adjustment to the inverting input (-) of deviation amplifier 22.
The conventional technique shown in FIG. 1, has the following deficiencies and disadvantages.
1. Because the frequency ratio converting circuit used to set the span of the converter comprises delay circuit, gate, and binary or decimal counter, etc, its configuration is complicated. In particular, since a high degree of accuracy is required increasingly in the field of electromagnetic flow meters, the setting of the span with a degree of precision not exceeding 0.1% is desired and a setting circuit which can handle a four digit decimal number is now needed. Thus, a span setting circuit becomes complicated if formed by use of counters, as is done in the prior art.
2. In the electromagnetic flow meter, the output signal of the detector varies in magnitude slightly from detector to detector. Thus, in order to prevent a span error from arising in case any detector and converter are paired, the meter constant representing the extent of variation of the signal voltage given by the detector is determined for each detector and the gain of the first stage amplifier of the converter is adjusted according to the meter constant peculiar to the incorporated detector. Under such circumstances, to adjust the gain of the amplifier 11, an analog voltage dividing circuit, such as a potentiometer is needed. Thus, it is difficult to provide a degree of precision which does not exceed the desired 0.1%.
With respect to the conventional technique used in FIG. 2, other deficiencies and disadvantages occur.
3. The span is changed by adjusting resistor 21. Thus, the degree of accuracy in setting the span depends upon the degree of precision of resistor 21. Accordingly, it is necessary to set the value of resistance accurately. However, this procedure has a distinct limitation in that it is an analog type of adjustment system.
4. Because of the foregoing procedure of setting the span, current output I.sub.o2 is required to be adjusted and made 0% when frequency F.sub.i2 is zero, and this current output I.sub.o2 does not decrease beyond 0%, even if resistor 29 is adjusted and its value of resistance is set to a value lower than that corresponding to 0%. Thus, it is difficult, with the prior converter, to adjust the zero point.
Thus, it can be appreciated that the prior art has a variety of deficiencies and disadvantages and leaves much to be improved upon.